Electron beam focusing magnet system for traveling wave tubes



June 13, 1961 E. DEIMEL 2,988,659

ELECTRON BEAM FOCUSING MAGNET SYSTEM FOR TRAVELING WAVE TUBES Filed June 4, 1959 FIGS INVENTOR ERNST DEIMEL AGEN United States Patent Claims priority, application Netherlands June 27, 1958 Claims. (Cl. 313-84) This invention relates to a magnet system for travelingwave tubes, which system comprises a number of radially arranged, substantially horseshoe-shaped magnets which link up with two bored polygonal soft-iron pole pieces, a stack of thin soft-iron plates of high permeability having about the same outer dimensions as the pole pieces and alternating with spacing rings of non-magnetic material being arranged between the pole pieces through the greatest part of the distance by which the pole pieces are spaced from one another.

Such constructions using horseshoe-shaped magnets are already known as are constructions without the intermediate soft-iron plates, in which instead of horseshoe magnets each component magnet comprises two radially arranged rods which are magnetized in opposite directions and are connected by a soft iron axial connecting piece.

In such constructions which, however, do not contain the soft-iron plates between the pole pieces, it is known to homogenize the field of axial direction between the pole pieces, that is to say, to make the strength of the field uniform throughout the entire length, by connecting the pole pieces to one another by means of a profiled soft-iron tube or of two broad parallel arranged profiled soft-iron plates.

This method has two disadvantages, namely: due to the connection between the pole pieces magnet flux for the field proper is lost and also the soft-iron plates which are disposed between the pole pieces for rendering the transverse fields rotation-symmetrical, can no longer be used unless pole pieces and magnets are employed which have abnormally large dimensions and abnormally high weights and prices.

It is already known for a magnet system for a travelingwave tube to comprise an approximation to an ellipsoid provided with a large bore and a soft-iron homogenizer of the longitudinal field within or without which soft-iron rings or plates are provided for rendering the transverse field rotation-symmetrical. However, this is a very bulky and expensive construction having a large stray field.

It is an object of the present invention to provide a construction which enables a homogeneous longitudinal field to be produced with the use of magnets of normal size and a rotation-symmetrical transverse field between the pole pieces.

In a magnet system for a traveling-wave tube comprising a number of radially arranged substantially horseshoeshaped magnets which register with two bored polygonal soft-iron pole pieces, with the interposition, between the pole pieces through the greater part of their spacing, of a stack of thin soft-iron plates of high permeability and having about the same outer dimensions as the pole pieces, which plates alternate with spacing rings made of nonmagnetic material, according to the invention there are arranged between the pole pieces a number of soft-iron rings, the diameters of which exceed the diameter of a stack and the cross-sectional areas of which decrease with increase of the distance from the nearest pole piece. This provides an approximation to the soft-iron connecting tube. However, the construction in accordance with the invention may also be regarded so that each of the rings in its axis produces a field of opposite direction to the main field. This field of opposite direction is stronger as A the cross-sectional area of the ring is larger, so that the rings nearest the pole pieces provide the strongest field of opposite direction, and this is exactly what is required to provide homogenization of the longitudinal field.

The simplest embodiment of the invention is that in which two soft-iron rings are provided close to each of the pole pieces, the ring nearest the pole piece having an internal diameter which is substantially equal to the longest diagonal of the pole piece and an axial dimension substantially equal to that of the pole piece and a radial width equal to one half of the axial dimension, while the second ring has the same external diameter as the first ring, one half of the radial width thereof and from one third to one half of the axial dimension thereof, the rings being spaced by a distance which is equal to one half of the axial dimension of the second ring.

The provision of the rings in accordance with the invention ensures that a sufiicient amount of space is left for the soft-iron plates for rendering the transverse field rotation-symmetrical, which field can extend nearly up to the pole pieces, the homogenization of the axial field being highly satisfactory so that this field midway between the pole pieces is even stronger than in the absence of the rings.

Preferably, the soft-iron rings in accordance with the invention nearest the pole pieces are provided with two diametrically arranged cut-away portions extending through about one half of the axial dimension so that they are shaped in the form of claws for the passage of the waveguides forming the input and the output of the traveling-wave tube.

At the end of the traveling-wave tube at which the electron gun is arranged and at which the number of iron component parts is less than at the collector end, according to the invention the ring nearest the pole piece may have a slightly smaller outer diameter through about one half of its length at the side adjacent the pole piece so that the radial width of the ring is reduced by about 25% at this point.

In order that the invention may readily be carried out, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is an axial sectional view of a magnet system in accordance with the invention with built-in travelingwave tube and high-frequency connections,

FIG. 2 is a front elevation of the magnet system and FIG. 3 is a sectional view of one of the homogenizing rings taken at right angles to the ring axis.

In the figures, reference numeral 1 designates the glass wall of a traveling-wave tube which at the gun end is sealed, with the interposition of a non-magnetic disc 2, to a glass wall 3 enclosing the gun. The emitting electrode of the gun is designated 4. The remainder of the gun is not shown. A soft-iron screening sleeve 5 surrounds the gun part. At the collector end, the glass wall 1 is sealed, With the interposition of a magnetizable ring Q, to a short glass cylinder 10 which terminates in a collector 8 provided with cooling fins and shown partially only. A transition part 9 and a screen electrode 10 are made of magnetizable material. Square pole pieces 11 each have a round aperture surrounding the ends of the tube. Each pole pieces engages four rods 12 of high-quality magnet steel which are magnetized according to the arrows. The rods 12 are interconnected at their outer ends by soft-iron connecting bars 13. The traveling-wave tube is accommodated in a brass screening sleeve 14 which terminates in red-copper input and output wave guides 15 and 16. The brass screening sleeve is surrounded, through its entire length between the input and the output wave guides, by rings 17 from mu-metal of thickness 0.2. mm. alternating with hardpaper rings 18 of thickness 3 mms. Near the pole piece at the gun end are provided a soft-iron ring 19 and a small ring 20 and likewise at the collector end rings 21 and 22. The rings are fixed relative to the pole pieces and to one another by means of three profiled hard-paper rings 23, 24 and 25 while two brass plates 26 determine the correct spacing relative to the pole pieces at each end. The ring 19 is provided externally with a brass sunk ring 27. The helical delay line of the traveling-wave tube is designated by 28.

FIG. 3 shows the provision of two cut away portions 29 in ring 21 so that two claws 30 are left.

What is claimed is:

1. A magnet system for a traveling wave tube comprising a pair of spaced, apertured, axially-aligned, poly onal-shaped pole pieces having an outside diameter defined by the longest diagonal across the pole pieces, a plurality of externally mounted permanent magnets opposite poles of which are coupled to the polygonal-shaped pole pieces, thereby establishing a magnetic field directed axially between the said pole pieces, and means for improving the uniformity of the said axially directed magnetic field, said improving means comprising a cylinder coaxially surrounding a substantial portion of the space between the polygonal-shaped pole pieces and comprising alternately arranged high-permeability magnetic wafers and non-magnetic spacers, said cylinder having approximately the same outside diameter as that of the said pole pieces, said improving means further comprising plural annular magnetic members coaxially surrounding the space between the pole pieces, said annular magnetic members possessing substantially greater outside. diameters than that of said cylinder, the annular magnetic member adjacent the nearest pole piece having a larger crosssection than further removed annular magnetic members.

2. A magnet system for a traveling Wave tube comprising a pair of spaced, apertured, axially-aligned, polygonal-shaped pole pieces having an outside diameter defined by the longest diagonal across the pole pieces, a plurality of externally mounted permanent magnets opposite poles of which are coupled to the polygonal-shaped pole pieces, thereby establishing a magnetic field directed axially between the said pole pieces, and means for improving the uniformity of the said axially directed magnetic field, said improving means comprising a cylinder 4 alternately arranged high-permeability magnetic Wafers and non-magnetic spacers, said cylinder having approximately the same outside diameter as that of the said pole pieces, said improving means further comprising a pair of annular magnetic members coaxially surrounding the space between the pole pieces and adjacent each pole piece, said annular magnetic members possessing substantially greater inside and outside diameters than that of said cylinder and surrounding the latter, the one annular magnetic member of each pair adjacent the nearest pole piece having a larger cross-section than the other annular magnetic member.

3. A magnet system as set forth in claim 2 wherein the said one magnetic member has an internal diameter substantially equal to the longest diagonal of a pole piece, an axial length substantially equal to that of the pole piece, and a radial width substantially equal to one-half its axial length, and the said other magnetic member has the same outside diameter as the said one member, onehalf of its radial width, and between one-third and onehalf of its axial length, the two members being spaced apart a distance equal to one-half of the axial length of the other member.

coaxially surrounding a substantial portion of the space between the polygonal-shaped pole pieces and comprising 4. A magnet system as set forth in claim 2 wherein one annular magnetic member adjacent a pole piece has a region adjacent thereto of reduced diameter.

5. A magnet system as set forth in claim 2 wherein the annular magnetic members adjacent the pole pieces have a laterally-extending channel forming opposed projecting portions.

References Cited in the file of this patent UNITED STATES PATENTS 

